Power supply for electron microscopes



Sept# 3, 1945 A. w. vANcE 2,406,974

POWER SUPPLY FOR ELECTRON MICROSCOPES Original Filed Nov. 15, 1940 3 Sheets-Sheet 1 M100 M 01PM/5e 1950 V.1 ffm/07250 Hauff@ cavas/r5.5? ca/L KI Bg @w i Gttorneg Sept 3 1946' A. w. vANcE Y 2,406,974

POWER SUPPLY FOR ELECTRON MICROSCOPES Original Filed Nov. 15, 1940 3 Sheets-Sheet 2 f @Mraz :inventor Bg @MMA attorney Sept. 3, 194e.

A. w. vANcE POWER SUPPLY FOR ELECTRON MICROSGOPES original Filed Nbv. 15, 1940 3 Sheets-Sheet 5 Zmventor Patented Sept. 3, 1946 POWER SUPPLY FOR ELEcfrRoN `N1IGROSCOPES Arthur W. Vance, Granbury, iN. J., assigner to Radio Delaware Corporaties pf America, a sommation 9i pplication April 28, 1942, Serial No. 440,721, "which is" a division of application Serial No. 365,750, Novembei` 15, 1940,\now Patent No. 2,302,900, dated November` 24, 1942. Iyidl and this application April 17.1943, Serial N0,-

The subject invention relates to DOWcI' Sui),- ply systems for electronic devices and has for its principal object the provision of an improved method and means for producing a highly s tabilized source of voltage for use with the elec,- tron microscope whereby greatly improved Der. formance may be obtained.

This application is a `division ,of applicant@ copending U. S. application, Serial Number 440,771, filed April 28, 1942, which is a Ydivision of U. S. application, Serial Number-365,750, filed November 15, 1.94.0, upon which U. S. Patent 2,302,900 was grantedto applicant November 2%, 19.42.

The development of the electron microscope has presented new problems in the `design of high voltage power supplies and in the design .of current regulating systems for the Various electro.- magnetic coils which constitute 4the vmagnetic lens system of the microscope. In order to produce a beam of -h-igh velocity electrons, an electron gun is utilized which is energized with a direct current voltage which may be as high as 100 kilovolts. The current load of the high voltage source is of the order of 1 milliampere, and the problem, therefore, is one of voltage stabil.- ity. The magnetic lenses require currents of the order of 400 milliamperes which likewise must be maintained constant over short periods.

The stability of the voltage supply system for an electron microscope is measured over the period of time required to properly expose a photographic plate used to record the images. In practice, the longest .exposure is approximately 30 seconds. Consequently, the voltage supply system must not vary appreciably within this period, since slight variations in the .electron speed or in the focus of the magnetic `elds will defocus the electron image and distort .the photograph.

It is also necessary to consider the short-time constancy of the power supply system since this limits the resolution of the microscope. `Theoretical calculations based on a resolving limit of 10 angstrom units indicate that the most critical current must not vary more than .004 of one percent and that the high voltage should not vary more than .005 of one percent.

1t is well known that the electron beam must be thoroughly shieldedfrom all .external fields. The difficulty of shielding magnetic fields produced `by A60 cycle currents has heretofore necessitated the use of direct current to supply power to the larnent of the high velocity electron gun. Since the cathode is normally ,opere 1 Claim. (Cl. 171.7312),

ated at a high negative potential, with the anode grounded, .the filament supply is necessariiy at a high voltage with `respect to ground. It lis a further object of this invention to eliminate the necessity iof supplying direct ,current for the electron gun by `energizing the liilament with radio frequency currents. A st-,ill further object or this invention is tp provide an improved high Voltage rectiiier circuit having a feedback stabilizer which permits the required high ycltage to be obtained with less equipmentand fewer parts than heretofore been required, thus greatly reducing the cost V and size of the power supply system. Still further objects nof the Apresent invention are to provide an improved automatic relay and protectivren u1,t,`to provide an improved oscillator Y plylng the radio frequency currents utill .energizing thehigh voltage rectifier; and to de improved current regulators for use in conjunction .with the microscope lenses.

Al'orieiiy, these objects .are accomplished hy employing a source lof .radio frequency currents coupled to a voltage doubling rectier circuit in which the --lanfrent of :oneof the rectier tubes is4 enengiz Y with radio frequency currents, thus facilitating fil-tering problems, 4deriving a potential corresponding in amplitude to the rectified outp A, and utilizing this potential to control the output of Ythe oscillator supplying the high fre* quency currents to the rectifying system.

nThis invention nwill be vbetter understood from the following descigption when considered in connection with the accompanying drawings in which Figure '1 is a `block diagram yof the essential elements of a compiete power supply system for anfelectron microscope.; Figure 2 is an equivalent lcircuit diagram illustrating the operation ofthe high voltage rectifier and filter system; Figure 3 is asimplied circuit diagram of the system including the high voltage rectiiier; Fi.,- ure fi is the circuit diagram .of the driver oscillatorwhich supplies lhigh yoltage radio frequency currents to .the high voltage rectifier; Figure 5 isthe circuit diagram ora D.C. modulating amplifier; Figure 64 is .a circuit diagram of a protectiye circuit `including an overload relay; vFigure f7 Ais the .circuit ,diagram of a .regulated current supply. suitablefor use in conjunction with ,the electromagnets .ofthe lens system; and Figure 8-gis Lanalternative current regulating system.

`vThe power supply system Referring to Fig. 1, a complete power supply system for an electron microscope ,is illustrated in blocktdiagram form. -Power isderived from a andere 3 60 cycle power line 9 and applied to a number of regulated power units which energize various components of the system and which are designed to have output voltages which are determined by their intended use. The regulated power supply Il energizes a pair of low frequency oscillators I3 and l5 which are utilized, respectively, to

the high voltage rectifier. A modulating amplifier 23 is energized through a connection 22 by the unregulated power unit i9 and is also supplied with a voltage over a lead 25 which is pro'' portional to the rectied output voltage. This voltage is compared to that supplied to the mod# ulated amplier by a standard battery 21, in a circuit which will be described in detail hereinafter, to produce an outputcontrol voltage which is utilized to control the vamplitude of oscillation of the driver oscillator 2|. This control connection is illustrated by lead 29. The ampliiier is also connected by a lead 20 to a regulated power source 3l which supplies a negative voltage of the order of 350 volts, the purpose of which is to be described later.

Another regulated power unit 33 supplies a direct voltage of the order of +400 volts to the driver oscillator 2! and to the modulating ampliiier 23 through a lead 35. In order to achieve the highest degree of accuracy, the modulating amplier 23 and, in addition, a current regulator ii for the objective coil are Supplied with a reguated current through connection 24 from a source 39 which is used to energize the filaments of certain of the tubes of these devices. Regulation in the current regulator 39 is achieved by utilizing the 350 volts obtained from the regulated power supply 3l by a connection 32 as a standard reference voltage, in a manner which will be described hereinafter. The projection coil and the condenser coil lll and d3 are energized by current regulators i5 and 61, which derive power from power supplies 42, 44, respectively, both regulators having a connection over conductor 32 to the regulated power supply 3l. A particular advantage of this system of interconnection is that one power supply whichis carefully regulated controls the output voltages of the other power supply units, and control by a regulated or standard voltage is eectuated in a much more economical manner than by any other method.

The high voltage rectifier Referring now to Fig. 2, a circuit diagram of the high voltage rectiiier system is shown. Since an extremely high voltage is produced, the high voltage equipment has been mounted in an oil tank l which not only protects the user from harm, but also decreases the danger of accidental flashover or short circuits. The driver oscillator 2| is connected to a series resonant circuit comprising an inductance Lo and a capacitance Co, the latter representing the effective lumped capacity between the point 54 and the ground due to the primary of transformer T2, and the rectiiier tubes Vl and V2 in series with the capacitor CI. The cathode of the first rectifier VI is grounded, while its anode is connected to the inductor` Lo through the coupling capacitor CI and also the cathode of the second rectifier V2. The'anode of the second rectifier is connected to the cathode of the high velocity electron gun of the microscope and is the source of high voltage direct potential whose polarity is negative with respect to ground.

Therilament of the rst voltage rectiier Vl is supplied from a conventional cycle line, while the filament of the high voltage rectifier V2 is obtained from thev filament oscillator E3 which is connected to input terminals E3, the circuit including a coupling transformer T2 having an untuned secondary and a tuned primary.

Radio frequency actuation of the high voltage rectifier has a number of advantages over the use of Ya conventional 60 cycle alternating power source for the production of high voltages of great constancy at low current drains. For example, much smaller iilter condensers are required for a given ripple output. This means that the stored energy inthe lter circuit is vvery much less than that of the usual 59 cycle brute force filter system. The importance of this is that in case of a, flash-over in the high voltage circuit, due to a loss of vacuum in the microscope, for example, the stored energy is not sufricient to burn up the equipment, as was vthe case in the earlier microscopes. Since Aresonant circuits are used, the ripple fed through the rectiiier capacitor is sinusoidal and consequently can be resonated out to a large degree. Furthermore, by using low loss coils in the resonantl circuit, an extremely high impedance may be realized which occupies but a small space and is extremely light in weight as compared to an equivalent impedance'at the conventional power frequency. Also the exciting power required by such a coil is greatly reduced.

The use of radio frequency high voltage power supply also improves the operation of the output control system.. When the control is operated on the low voltage input side of the rectiiier,-as is desirable, the speed of control is limited by the frequency of the supply. Consequently, this limitation is negligible Vwhere the supply voltage is a radio frequency voltage.

The use of radio frequency to energize the cathode 0r lilament of the high velocity electron gun has two important advantages. In the 'first place, the microscope may be readily shielded from the stray high frequency elds which are produced, and this result is aided by the fact that no 60 cycle high voltage transformers are required which have very large external iields. In the second place, the use of radio frequency eliminates the necessity for employing bulky storage batteries or their equivalent.

The operating frequency for the rectiiier s-ystem is not critical. The problem is essentially that of obtaining the required output voltage with a minimum exciting power. This, of course, requires obtaining the maximum resonant impedance of the high voltage coil. The resonant impedance is given by the formula:

where C is the distributed capacity of the coil plus the rectier interelectrode capacities plus other stray capacities such as capacity from the primary to the secondary of the iilament transformer T2. It is obvious that C should be kept to a minimum,` and with C at a minimum, Z may s. only be raised by decreasing f or by increasing Q, where Q is the well known efficiency factor:

and R1. and Rc are the effective series A.-C. resistances of L and C, respectively. In the frequency region from approximately 20 kc. up to several kundred kc., the maximum Q possible in a coil of given volume is more or less independent of frequency; thus, the coil should, in general, operate below 50 kc.

The rectifier circuit is seen to be of the type wherein the inverse voltage of the half wave rectifier VI is rectified by V2, thereby charging the output capacitor C2 to a voltage nearly equal to twice the peak voltage which appears across the induct- Y,

ance Lo. It is to be noted that one side of the input circuit is grounded and that no primary winding is required on the high voltage coil.

The filament transformer T2 has a low interwinding capacity which is preferably of low power factor. The primary and secondary windings must be spaced sufficiently to withstand the peak voltage output, and, as a result, a large amount of energy must be stored in the tuned primary in order that the secondary may absorb the power necessary to excite the filament. Preferably, the tuned primary of transformer T2 constitutes the tank circuit of the driving filament oscillator. The size of the secondary is selected to match theI impedance of the filament load and to provide the desired voltage. The filament current may be controlled conveniently by varying the frequency of the oscillator I5.

Fig. 3 shows the rectifier circuits connected to the other elements of the power supply system. It will be noted that the negative voltage for the electron gun cathode obtained from the output of rectifier V2 is applied to a center tapped resistor 51 connected across the secondary of the transformer T3 which supplies radio frequency current to the filament of the electron gun 59. The filan ment, as a whole, is 60 kilovolts below ground potential and must therefore be carefully insulated. The radio frequency current supplied by the filament oscillator I3 is applied to the filament through a coaxial cable 6 I.

It will be noted that the filter capacitor C2; .005 microfarad, for example, is connected to ground through a parallel connected coil L5 and condenser C5. The values of these reactors are such that at the operating radio frequency, effective inductive reactance of L5 and C5 resonates with capacitor C2 and forms an effective ground for ripple voltages appearing in the output circuit. At a higher frequency, for instance, midway between the fundamental and second harmonic L5 and C5 become parallel resonant, presenting a highimpedance, but as there is no ripple at this frequency anyway, no harm results. At higher frequencies including the vsecond harmonic, the shunt capacitor C6 in series with the output capacitor C2 provides an effective ground. A spark gap 63 is connected across the shunt reactors L5 and CE in order to discharge voltage surges Without causing a breakdown of the elements.

High voltage regulator The amplitude of the rectied direct current is controlled by a circuit including the D.C. modulating amplifier 23 which controls the driver oscillator 2|, a voltage divider comprising a high resistance capacity-compensated high voltage 'resistor E5, 1100 meg., and a relatively low resistance 6 resistor 61, 10 meg., serially connected between the high voltage source and the regulator. The lower end of resistor 61 is bypassed to ground and connected through an isolating resistor to an adjustable tap on a standard battery 69, the

negative terminal of which is grounded. The standard voltage produced by the battery 69 is therefore compared to the divided voltage appearing across resistor 6l, so that no input voltage is applied to the amplifier 23 when the divided Voltage obtained from the high voltage output is exactly equal to that of the standard battery. By varying the taps on the standard battery 62, the output voltage may be controlled, for example. in 5 kv. steps from 30 to 60 kvs.

The details of the driver oscillator which supplies high voltage for the rectifier tubes VI and V2 is shown in Fig. 4. Since the resonant frequency of the series resonant inductor Lo and Co included Within the oil tank 5I varies considerably with temperature, a master oscillator-power amplifier is impractical without automatic frequency control to keep it at resonance, It is therefore proposed to utilize a self-oscillating circuit-l whose frequency is determined wholly by the resonant frequency of the load Lo, Co. This oscillator must also be capable of modulation over a considerable range in order to provide control over the output voltage. The circuit utilized comprises a two-stage oscillator including tubes V3 and Vfl, the output of the latter being coupled to the input of the former by a conventional impedance cou-4 pling system, tube V4 being shunt fed through inductor L4 and coupled to the resonant circuit load through a small series resistance Pt. Inductor L4 is resonated at the operating frequency by a capacitor C5. A feedback voltage is obtained by the drop across resistor R., the terininais of which are connected to the primary of a shielded transformer 'I I, the secondary of which is coupled to the input of the first oscillator tube V3. The transformer 'il preferably has a broad frequency response, this being accomplished, for example, by a damping resistor and capacitor '3.3 and l5. The feedback gain is sufficient only to sustain oscillation at or very near the resonant frequency of the load Lo, Co.

The amplitude of oscillation is controlled by varying the screen grid potential of the output tube V4. This is illustrated in Fig. 4 by the potenY tiometer connection shown in dotted lines, the potentiometer representing the control bias derived from the D.C. modulating amplifier 23. The details of this connection are illustrated in Fig. 5. The tank circuit L4, C5 has a high L/C ratio and it therefore exerts only slight control over the frequency of oscillation. Its principal function is to maintain reasonably sinusoidal voltage conditions in the plate of V4.

Referring to Fig. 5, a unique D.C. amplifier is illustrated suitable for use in applying the small variable D.C. voltage of the control sys tem to the driver oscillator to effectuate control of the amplitude of oscillation. Resistor 51 corresponds to the similarly numbered resistor in the voltage divider circuit of Fig. 3. At any instant, the potential of the lead 22 connecting this resistor to the control grid of the first arnpliiier tube 'll is equal to the sum of the negative divided voltage and the voltage due to the standard battery 69. The latter battery is preferably tapped in steps and constitutes the main control of the high potential output.

The filament of the rst amplifier tube 'l1 is connected to a regulated current source, which may be of the type illustrated in Fig. 8 and hereinafter described, in order to assure constant electron emission. Screen grid potential is obtained from a voltage divider 19-8l connected to a suitable source of positive potential which is applied to terminal 83. Plate voltage is obtained from a resistor |05 connected between the plate and the terminal 83. The plate is connected to the control grid ofV the second amplifier tube 85 through a parallel connected resistor 81 and capacitor 89. A phase control network comprising vresistor 9| and capacitor 93 is connected in shunt with the plate of the amplifier tube 11. Screen grid potential for the second amplifier tube 85 is derived through a dropping resistor 95 from a suitable source of positive potential which is connected to a terminal 91. A gas lled regulator tube 95 is utilized to control the screen grid potential. The anode of the output tube 85 is connected through an anode load resistor to the positive supply terminal 83 and also through an isolating resistor |03 to the screen grid of the second oscillator tube V4, which is shown in Fig. 4.

It will be noted that the plate voltage of the first amplier tube 11 is impressed on the grid of the second amplifier tube 85 through the coupling resistor 81. Since the cathodes of the two ampliiier tubes are both operated at ground potential, it is necessary to overcome the effect of the plate voltage on the grid of the second ampliiier. This is accomplished by connecting a source of negative voltage, for example, -350 volts derived from the voltage supply 3| illustrated in Fig. 1, through an isolating resistor |91 to the control grid of the output tube. This has an effectl similar to that of the usual series bucking battery in D.C. ampliers, but has the advantage that one terminal of the source of negative voltage is operated at ground potential, which is'not true in the conventional case.

It has also been found that it may be desirable to drive the screen grid of the oscillator tube more negative than is possible by direct control from the amplifier tube 85. This is accomplished by connecting the -350 volt source to the lead |09 through a resistor thus reducing the average potential of the screen and making it possible to drive the screen more negative when the output tube 85 is drawing maximum plate current.

Protective circuit den current surges and limits the current to substantially the normal value. In order to pro-v tect the apparatus from sustained overloads,` a cutout or overload relay 69 hasbeen provided. The details of the protective circuit and overload relay are illustrated in Fig. 6, to which reference is now made. Y,

Transformer corresponds to the'similarly numbered transformer of Fig. 3 which supplies 60 cycle alternating current to the first high voltage rectifier tube VI. made through resistors ||1, H9, |2|, meter A, resistor |23, the energizing coil of an overload relay |25 and a resistor |21. A neon regulator tube |29 and a pair of condensers |3| and |33 are connected between theoutput terminals of the meter A and the junction points of the resistors ||1, ||9 and` |2I, respectively. The output terminal of the meter A is. also connected'to ground throughy a limiting neon tube |35.

The overload relay circuit includes a switch |39 which, in its normal position, makes contact to a grounded terminal |4I. When actuated by an overload cur-rent, the relay armature makes contact to a terminal |43 which is connected to a source of negative voltage, for example, the -350 volts provided by the power supply 3|. The armature is connected to the grid return of the oscillator tubes of the driver oscillator as illustrated in detail in Fig. 4. The armature is also connected through a push button |41 to the relay coil through a current limiting resistor |49. The same armature is also connected to a voltage divider |5| across a portion of which a neon indicator tube |53 is connected.

A current surge, caused by an arc or breakdown of a high voltage termi-nal, flows through the limiting resistors H1, ||9 and |2| and charges capacitors |3| and |33. In addition, the voltage across the output meter A is limited by the neon tube |29. The meter is therefore protected from damage. The sustained overload flows through the actuating coil of the relay |25 and connects the armature to the negative potential source, thus Vremoving the normal ground connection and applying a high negative potential to the driver oscillator sufcient to stop its oscillation. Since the driver oscillator provides the high voltage forvthe highV voltage rectifier system, it will be apparent that the relay immediately shuts off the high voltage sup` ply. At the same time, the relay connects a holding circuit through the push button |41 sov that current flows through a resistor |55 and through the relay to hold it in its closed posinon. It will remain m this position untu the circuit is broken by operati-ng the push button |41. If the fault has cleared, the overload relay will open, restoring the high voltage to the rectiers. VIf the fault has not cleared, the high voltage will not remain on,- indicating that thev circuit must be checked.' When the overload relay is actuated, the neon output indicator |53 will light, thus providing a visual indication to4 the operator that the high voltage has been cut olf.

Low voltage or current regulators A current regulator of the type preferred for use with the projection and objective coils of the microscope is illustrated in Fig. '7. A conventional rectifier |51 and iilter |59 supply a high voltage to the plate of a triode IGI, the cathode Yconnected to an input terminal |13 through a plate resistor |15. The plate of this control tube is connected to the grid'of the through acoupling resistor |11.

The ground return istriode IGI It will be observed that the cathode of the triode ISI is positive with respect to ground. However, the plate of tube ITI cannot become suiiiciently less positive than the cathode of tube IGI to provide the necessary bias for the latter tube under conditions of low output current. In order to supply the proper voltage to the grid of the tube IBI, it is connected through an isolating resistor I 'I9 to the source lof regulated negative voltage, as in the case of the D.C. amplifier described above.

A current regulator of somewhat simpler form which does not require a standard battery is illustrated in Fig. 8. This amplifier is preferably used for the condenser coil of the microscope and may also be used to regulate the filament current of the D.-C. amplier and the control tube I'II ofthe current regulator illustrated above in Fig. '7. The unit may also be considered as a voltage regulator since the voltage across a constant impedance is constant when the current through it is constant. Consequently, the same control circuit is employed, for example, in regulating the output of the regulated 800 volt power supply Il.

As before, the conventional rectier and lter I'I and I 59 are connected to the plate of a limiting triode IBI, the cathode of which is connected to ground through the electron microscope condenser coils or the regulated filament, as the case may be, and through the control and adjusting resistors |65 and |61. The voltage drop across the latter resistors is compared to a voltage derived from the regulated -350 volts obtained from the power supply 3|, thus eliminating the standard batteries |69 utilized in the preceding regulator. A particular advantage of thismethod is that the standard voltage source is operated with one terminal grounded, which is not possible where standard batteries are used in a series circuit. Since the cathode of the triode IGI is positive with respect to ground, as noted above, it may not be necessary to apply the auxiliary negative potential to the grid of this tube so that in this case the plate of the control tube I'II is connected directly to the grid of the triode I6I. It will be apparent that in both cases, changes in the current through the coil and control resistors produces a variable voltage drop across the resistors which is applied to the control tube 10 ITI to vary the amplitude of the current in a direction tending to compensate, for the change.

The measured stability of the power supply system herein described has been found to be well in excess of that required for stability and definition of an electron microscope. Several hundred photomicrographs have been successfully taken with no indication that the results have been limited by variations of the power supply. This is in distinct contrast with microscopes of the prior art in which a large percentage of the photomicrographs is spoiled by reason of changes in the power supply voltage. The system is so stable that the microscope may be reset to previous conditions without observing the electron image or the focusing. Exposures thus made have good resolution. In addition, it is possible to make wide variations of the image intensity Without changing the focus.

A particularly severe test which was successfully passed by the electron microscope operated in conjunction with the power supply and control system of the present invention is that of reducing the vacuum during operation until an internal arc takes place, causing the overload relay to actuate, then restoring the vacuum, and applying the high voltage to obtain the original picture exactly in focus without readjustment. Previously known microscopes could not be treated in this manner and generally had to be taken apart and repaired after an internal flashover due to the severity of the discharge from the filter.

I `claim as my invention:

In a stabilized power supply system including a plurality of separate direct current power supply units energized by a common power line, one of said units having its positive terminal grounded and the other units having their negative terminals grounded, regulating means for stabilizing the negative voltage output of said one unit, means for applying said negative voltage to other units of said system, means for comparing the positive output voltages of said other units with said negative voltage to obtain a dierence voltage of substantially zero amplitude, and means responsive to changes in the amplitude of said di'erence voltage for controlling the output voltages of one of said other units.

" ARTHUR W. VANCE. 

